The duplicate invoice check does not detect a duplicate invoice. Not all fields have received status "complete" in verify (not all fields are green when opening the invoice in verify).
The requirements of the duplicate check function have not been fulfilled.
Info from the help file:
"Only those invoices, for which all fields receive Complete status, are considered as potential duplicate invoices. In addition, if any field on an invoice has more than one proposed value, that invoice is not considered as a potential duplicate invoice even if the field receives Complete status."
For further information please also see the link under references.
Look into if you can optimize the invoice definition so that all fields are complete (green) when opening the invoice in verify.
Level of Complexity
Automatically identify duplicate invoices during processing: